Method and system for providing a DC voltage with low ripple by overlaying a plurality of AC signals

ABSTRACT

An AC-DC converter that includes a plurality of power conversion units that overlay their signals to generate a DC signal with low voltage level, small voltage and current ripples, but high DC current. The DC signal is further filtered by a LC low pass filter to further minimize the ripples before output to the load. In one embodiment, the plurality of power conversion units is preferably powered by a plurality of input pulse generators. In an alternative embodiment, the power conversion units is powered by a input pulse generator circuit that includes a plurality of n-channel MOSFETs arranged in a full bridge configuration whose gate voltages are controlled by a plurality of pulse generators.

FIELD OF THE INVENTION

The present invention relates generally to an AC-DC converter and, morespecifically, to an AC-DC converter that produces a DC signal with smallripples.

BACKGROUND

FIG. 1A illustrates a typical AC-DC converter, which includes a powerconversion unit, an inductor-capacitor low-pass filter, and an inputpulse generator. The power conversion unit includes a transformer with aprimary and a secondary winding and two rectifier diodes with theiranodes coupled to either prongs of the secondary winding respectively.The cathodes of the diodes are, in turn, connected to each other as wellas to the low-pass filter. The primary winding is connected to the inputpulse generator.

In operation, the input pulse generator outputs an AC signal,illustrated in FIG. 1B, across the primary winding. The signal acrossthe primary winding is then transferred to the secondary winding, wherenegative portion of the signal is rectified by the rectifying diodes toyield the signal illustrated in FIG. 1C. This signal is then filtered bythe low-pass filter to output a DC output signal that includes outputripples, as depicted in FIG. 1E.

Although output ripples are only small perturbations when output DCvoltage level is high, the ripples become a prominent feature when theoutput DC voltage level is low. Since a variety of modem semiconductordevices such as microprocessors typically operate at low DC voltagelevels, the ripples may interfere with the proper operation of thedevices. Typical modem semiconductor devices have a tolerable ripplelimit of about 1% of the DC voltage. Voltage ripple is of increasedconcern for semiconductor devices that operate at lower DC voltages, forexample 1 V, because the absolute tolerable ripple decreases as theoperating voltage decreases.

One way to minimize output ripples is by improving filteringcapabilities of the converter. This may be achieved by increasing thecapacitance and/or lowering the impedance of the capacitor in thelow-pass filter. Real capacitors having a low series impedance ortypically expensive. However, large capacitors take up more space andlow impedance capacitors are expensive. Alternatively, a larger inductormay be used to improve filtering capabilities. However, a largerinductor also takes up more space. In addition, they saturate mucheasier.

Another way to reduce output ripples is to increase the frequency of theinput signal. However, high frequency current flowing through theinductor produces high frequency flux change in the inductor ferrite,which increases core loss and decreases efficiency of the inductor. Inaddition, inductors with low loss ferrite material is expensive.

Some work has been done to reduce output ripples by modifying thetypical AC-DC converter circuit. For example, U.S. Pat. No. 5,668,464 toKrein et al., which is incorporated herein, claims an AC-DC convertercircuit that incorporates a feedback control circuit that generates anAC ripple signal to cancel out output ripples. Drawbacks of thisconverter circuit are that the feedback control circuit not only addscomplexity to the circuit but also takes up precious space within smallsemiconductor devices. In addition, the output inductor carries large ACcurrent ripples that can degrade the inductor. Another patent, U.S. Pat.No. 5,663,876 to Newton et al., which is incorporated herein, describesa rectifier circuit with two output inductors that can produce a DCsignal without any output ripples. However, this can only be achieved byusing inductors with specific inductance values operating at apredetermined operating condition. In addition, current and voltageripples across the two output inductors are large and can degradingtheir performance.

Therefore, there is a need for an improved AC-DC converter that producesa DC signal with low DC voltage level, small voltage and currentripples, but high DC output current without large capacitors and/orinductors, addition of complex circuits, or strict operatingrequirements.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an AC-DCconverter that produces a DC signal with low voltage level and smalloutput ripples without large capacitors and/or inductors, addition ofcomplex circuits, or strict operating requirements.

It is another object of the present invention to provide an AC-DCconverter that produces a DC signal with low voltage level, smallvoltage and current ripples across the output inductor, but high DCoutput current without large capacitors and/or inductors, addition ofcomplex circuits, or strict operating requirements.

Briefly, the present invention provides an AC-DC converter that includesa plurality of power conversion units that overlay their signals togenerate a DC signal with low voltage level, small voltage and currentripples, but high DC current. In a preferred embodiment, the AC-DCconverter comprises a plurality of power conversion units, an inputpulse generator system, and a low pass filter. Each power conversionunit preferably includes a transformer with a primary and a secondarywinding, a DC blocking capacitor connected to the primary winding, and arectifier diode with its anode connected to the secondary winding. Thecathodes of the rectifier diodes are connected to one another as well asto the low pass filter, thereby connecting each power conversion unit toeach other as well as to the low pass filter. The low pass filter ispreferably an inductor-capacitor low pass filter. The input pulsegenerator system preferably includes a plurality of input pulsegenerators, each connected to the DC blocking capacitor of a powerconversion unit.

In operation, each input pulse generator preferably outputs a squarewave input signal that is the same as but out of phase with signalsgenerated by other generators, so that the signals overlap and at leastone of the signals is high at any given moment. In one embodiment, thesquare wave signals are evenly phase-shifted with respect to oneanother. Each input signal is transmitted to a primary winding through aDC blocking capacitor, which filters away any DC biases in the signal,and is then transferred from the primary winding to the secondarywinding. The signal at each secondary winding is then transmitted to arectifying diode, which rectifies the negative portion of the signal.Each rectified signal is then overlaid to generate a DC signal with lowDC voltage level, small voltage and current ripples, but high DCcurrent. The DC signal is then filtered by the low pass filter tofurther reduce voltage and current ripples to generate the DC outputsignal.

In an alternative embodiment, the AC-DC converter includes the samepower conversion unit and low pass filter as the converter describedabove but with a modified input pulse generator system. The modifiedinput pulse generator system preferably includes a first, second, third,and fourth n-channel MOSFET arranged in a full bridge configuration,where the drains of the first and third MOSFET are connected to eachother and to ground, the sources of the same MOSFETs are connected tothe drains of the second and fourth MOSFET respectively, and the sourcesof the second and fourth MOSFETs are connected to a DC voltage sourceand a DC blocking capacitor. The DC blocking capacitor is also connectedto a first and second power conversion unit, where the first and secondprimary winding of the first and second power conversion unit eachconnect to the DC blocking capacitor on one prong and to the sources ofthe first and third MOSFET respectively on the other prong. Preferably,a first, second, third, and fourth pulse generator are connected to thegate of the first, second, third, and fourth MOSFET respectively. Thefirst and second pulse generators output square pulses to alternatelyswitch the first and second MOSFETs on; that is, either the first or thesecond MOSFET is switched on, but never at the same time. A brief periodduring which both MOSFETs are switched off is inserted in betweenalternately switching of the MOSFETs to prevent connecting the DCvoltage source to ground. By alternately switching the first and secondMOSFET on, the first primary winding is alternately connected to the DCvoltage source and ground, generating a signal across the winding.

The third and fourth pulse generators output the same signals as thefirst and second pulse generators respectively but phase-shifted by 180degrees. This generates a signal across the second primary winding thatis the same signal as that across the first primary winding butphase-shifted by 180 degrees. The signals across the first and secondprimary winding preferably stay high longer than they are low, so thatthe two signals overlap while they are high and at least one signal ishigh at any one moment. The signals at the primary windings are thentransferred to the secondary windings and then to the rectifying diodes,where negative portions of the signals are rectified. The rectifiedsignals are then overlaid to create a DC signal with low voltage levelwith small voltage and current ripples but high DC current. The overlaidDC signal is then filtered by the inductor-capacitor low pass filter tofurther minimize ripples before output to a load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a typical prior art AC-DC converter;

FIG. 1B is a graph of the input signal generated by the input pulsegenerator across the primary winding of the converter depicted in FIG.1A;

FIG. 1C is a graph of the signal depicted in FIG. 1B after it istransferred from the primary winding to the secondary winding and isrectified by rectifying diodes of the converter depicted in FIG. 1A;

FIG. 1D is a graph of the current that flows through the inductor of theconverter depicted in FIG. 1A;

FIG. 1E is a graph of the output DC signal of the converter depicted inFIG. 1A;

FIG. 2 is a schematic diagram of a preferred AC-DC converter accordingto the invention;

FIGS. 3A-C are graphs of signals generated by input pulse generators ofthe converter depicted in FIG. 2;

FIGS. 3D-F are graphs of signals across the secondary windings of theconverter depicted in FIG. 2;

FIG. 3G is a graph of the signal at node B of the converter depicted inFIG. 2;

FIG. 3H is a graph of the current across the output inductor of theconverter depicted in FIG. 2;

FIG. 3I is a graph of the output DC signal of the converter depicted inFIG. 2;

FIG. 4 is an alternative embodiment of the AC-DC converter according tothe invention;

FIGS. 5A-D are graphs of signals generated by pulse generators of theconverter depicted in FIG. 4;

FIGS. 5E-F are graphs of signals across the secondary windings of theconverter depicted in FIG. 4;

FIG. 5G is a graph of the signal at node B of the converter depicted inFIG. 4;

FIG. 5H is a graph of the current across the output inductor of theconverter depicted in FIG. 4; and

FIG. 5I is a graph of the output DC signal of the converter depicted inFIG. 4;

FIG. 6 shows an embodiment of the present invention using sinusoidalinput signals; and

FIG. 7 shows an embodiment of the present invention using a singlesource of time varying input signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates a preferred embodiment of an AC-DC converter 10 inaccordance with the present invention. As shown in FIG. 2, AC-DCconverter 10 preferably includes a plurality of power conversion unitsthat each includes a transformer 100 with primary winding 110 andsecondary winding 120, a DC blocking capacitor 300 connected to primarywinding 110, and a rectifier, such as a rectifier diode 400 connected tosecondary winding 120. It should be understood that the transformers maybe replaced with inductors for non-isolation of AC-DC converter 10.

Upon reference to the present specification, one of ordinary skill inthe art would understand that although three power conversion units aredepicted in FIG. 2, fewer or additional units may be added with propermodifications. Increasing the number of power conversion unitspreferably decreases the relative amount of ripple present in the DCoutput voltage. More power conversion units may be added to anyembodiment of the invention to decrease voltage ripple and increase DCcurrent output.

An input pulse generator 200 is connected to each DC blocking capacitor300. The cathodes of diodes 400 are preferably connect at node B so thatsignals from each power conversion unit are channeled to and overlaid atthe node. Overlaying signals from a plurality power conversion unitsadvantageously provides a DC signal with low voltage level, smallvoltage and current ripples, but high DC current, as discussed below inconjunction with FIG. 3G. An inductor-capacitor (LC) low-pass filterformed by inductor 500 and capacitor 600 connects to the cathodes ofrectifying diodes 400 at node B. The output prong of inductor 500 of theLC low-pass filter supplies the output DC signal to load 700.

Various aspects of the operation of AC-DC converter 10 are depicted inFIGS. 3A-H. FIGS. 3A-C, illustrate signals generated by input pulsegenerators 200. Each input pulse generator 200 preferably provides atime-varying electrical signal. Thus, input pulse generators 200generate preferably periodic waveforms such as square waves ofpreferably equal magnitude and pulse width that are staggered evenly intime, so that, at any one moment, at least one of the input signals ishigh. That is, the time-varying electrical signals provided by inputpulse generators 200 are out of phase.

The waveforms output by pulse generators may have a constant or varyingfrequency. A non-limiting range of suitable frequencies would includefor example frequencies in the range of about 25 kHz to about 1 mHz,such as from about 100 kHz to about 500 kHz. Upon reference to thepresent specification, one of ordinary skill in the art would understandthat the values, such as the capacitance and inductance, of thecomponents of AC-DC converter 10 depend upon the frequencies of thewaveforms received from the pulse generators.

Each input signal is transmitted to a primary winding 110 through a DCblocking capacitor 300, which reduces and preferably blocks any DC biasin the signal to prevent saturation of transformers 100. Each signal isthen transferred from primary winding 110 to secondary winding 120,which transfer prepares time-varying output signals.

The time-varying output signals at secondary windings 120 (node A) areillustrated in FIGS. 3D-F. These signals are shaped by impedances ofcapacitors 300 and primary windings 110 resulting in sawtooth-likewaveforms that are slightly voltage-shifted downwards. The time-varyingsignals at the secondary windings 120 have a smaller absolute DC biasthan the input signals accepted by AC-DC converter 10. By absolute DCbias, it is meant the absolute value of the DC bias.

The time-varying output signals at secondary windings 120 (node A) arerectified by diodes 400 so that only the positive portions of thesignals remain. After rectification, signals from each power conversionunit are channeled to and overlaid together at node B to result in thesignal depicted in FIG. 3G. As can be seen in the figure, before asignal from one power conversion unit can decay to any significantextent, a second signal from another power conversion unit is overlaidon top of the first signal, resulting in a DC signal with minimumvoltage and current ripples. In addition, overlaying of signals allowseach power conversion unit to contribute to the total current at node B,increasing current level of the signal at the node and at the converteroutput. The rectifiers may be reversed to obtain a negative DC outputvoltage.

In an alternative embodiment, rectifying diodes 400 may be replaced withelectronic switches, such as, for example, MOSFET or synchronousrectifiers. A synchronous rectifier is provided by replacing a diodewith a MOSFET. For example, diode 400 may be replaced by a MOSFET havinga proper gate driving sequence. The gate driving voltage of the MOSFETis programmed to turn on when the circuit path, in which the MOSFET islocated, is under forward current. A MOSFET may have a voltage drop ofless than about 0.6 V, which voltage is typical of diodes.

Returning to FIGS. 3A-3I, the signal at node B is next filtered by a LClow-pass filter formed by inductor 500 and capacitor 600 to furtherreduce voltage and current ripples, yielding an output DC signaldepicted in FIG. 3I. Besides serving as a filtering element forminimizing ripples, inductor 500 also acts as a defined equivalent DCcurrent source for load 700 with current equal to the DC output currentof AC-DC converter 10. With a defined current load, input pulsegenerators 200 would be able to generate input current waveforms inphase with input pulse signals. In addition, a defined current sourceload minimizes numerous undesired circuit behavior such as resonancebetween leakage inductance of the transformer or undefined capacitanceon the load side. A defined current source load also draws smaller a RMScurrent from diodes 400 and input pulse generators 200, thereby reducingsignal loss caused by current peak. Diodes 400 or pulse generators 200preferably experience a square-wave like current waveform, whichprovides a minimum RMS value as compared to the average value of thecurrent. The waveform shape is determined a square wave by the inductorrather than by the unpredictable load current. The average value of thecurrent is determined by the load.

Referring to FIG. 3G, the voltage ripple at B does not go to zero as itdoes in prior art voltage supplies where the voltage ripple at theoutput inductor is a switching square wave. Because the ripple at B issmall, output inductor 500 does not require a large inductance.Preferably, the inductance of inductor 500 is selected to provide adefined current source load to pulse generators 200 and diodes 400.Because output inductor 500 is much smaller than prior art filterinductor, a suitable inductor may be parasitic inductance from thecircuit such as printed circuit board trace inductance.

The magnitude of the output voltage is determined by the amplitude ofthe input pulses, the duty cycle of the pulse width, and the turn ratioof the isolation transformers. For example, an increase in the amplitudeof the input pulses would increase the amplitude of the DC outputvoltage, as would a decrease in the turn ratio of the isolationtransformers. Here, the turn ratio is defined as Nprimary/Nsecondary,where N is the number of turns of the primary and secondary windings. Asthe number of secondary turns, Nsecondary, increases, the turn ratiowill decrease, thereby increasing the output voltage.

An AC voltage waveform, such as a square wave, applied to a transformerpreferably has a DC component of zero. For a voltage waveform having azero DC component, the positive area under a voltage v time curve of thewaveform is equal to the absolute value of the negative area under thevoltage v time curve. That is, the integral of the positive area of thewaveform is equal to the absolute value of the integral of the negativearea. When the duty cycle of the input pulses is varied, the AC voltageproduced by the transformer preferably varies in such a way as toprovide a constant voltage time integral. By constant time integral, itis meant that the integral of the positive area remains equal to theabsolute value of the negative area of the waveform. Additionally, it ispreferred that the peak to peak voltage of the waveform also remainsconstant as the duty cycle varies. For example, if the absolute value ofthe peak negative amplitude decreases, the peak positive amplitudepreferably increases to maintain a constant peak to peak voltage. Thisproduces variation of the positive amplitude after rectification and,thus, the DC output voltage.

AC-DC converter 10 according to the invention can be modified into anAC-DC converter 20 illustrated in FIG. 4. The modified AC-DC converterincludes an input pulse generator circuit, two power conversion units,and a capacitor-inductor low pass filter. The input pulse generatorcircuit preferably accepts a plurality of input signals from pulsegenerators 230 to thereby prepare intermediate time-varying signalsapplied to the power conversion units. The power conversion units outputtime-varying signals that are combined to prepare a direct currentoutput signal.

The input pulse generator circuit includes a plurality of field effecttransistors, such as four n-channel MOSFETs 210A-D, DC voltage source240, and pulse generators 230 A-D. N-channel MOSFETs 210 A-D arearranged in a full bridge configuration with drains of MOSFETs 210 A andC coupled to a DC voltage source 240 and their sources coupled to thedrains of MOSFETs 210 B and D, respectively. The sources of MOSFETs 210B and D are coupled to ground and DC blocking capacitor 300. Each gateof MOSFETs 210 A-D is coupled to a pulse generator 230 A-D that are eachprogrammed to output pulses of specific period and amplitude asdescribed in conjunction with FIGS. 5A-D below. The MOSFETs preferablyoperate as switches toggling between the on and off states as describedbelow. Thus, p-channel MOSFETs, which may also be used to perform aswitching function, may be used in an AC-DC converter of the invention.

Input signal generator circuit supplies input signals to two powerconversion units. Each power conversion unit includes a transformer 100and a rectifying diode 400. The two power conversion units also shareone DC blocking capacitor 300, eliminating the need for additionalcapacitors. Primary windings 110 A and B are coupled to nodes C and D,respectively, and to DC blocking capacitor 300. Secondary windings 120are each coupled to diodes 400. The cathodes of diodes 400 are connectedto node B so that signals from each power conversion unit are channeledto and overlaid at the node. Node B is, in turn, connected to a LClow-pass filter formed by inductor 500 and capacitor 600. The outputprong of inductor 500 of the LC low-pass filter outputs the resultant DCsignal to load 700. It should be understood that the transformers may bereplaced with inductors for non-isolation of AC-DC converter 20.

Various aspects of the operation of AC-DC converter 20 are illustratedin FIGS. 5A-I. The first four figures, FIGS. 5A-D, depict signalsgenerated by pulse generators 230 that are applied to gates of MOSFETs210A-D. Signals from pulse generators 230A and 230C are identical butphase shifted by 180 degrees. Similarly, signals from pulse generators230B and 230D are also identical but phase shifted by 180 degrees.Timing of the pulses is crucial for minimizing ripples in the DC outputof AC-DC converter 10, as discussed below in conjunction with FIG. 5G.

When the signal at pulse generator 230A is high and the signal at pulsegenerator 230B is low, MOSFET 210A is switched on while MOSFET 210B isswitched off, connecting primary windings 110A to DC voltage source 240at node C. With capacitor 300 at a lower potential than DC voltagesource 240, a potential drop is established across primary winding 110A,causing current to flow across primary winding 110 A to charge capacitor300. As capacitor 300 charges, the potential difference across primarywinding 110 A decreases.

When the signal at pulse generator 230A is low and the signal at pulsegenerator 230B is high, MOSFET 210A is switched off and MOSFET 210B isswitched on, connecting primary winding 110 A to ground through node C.The voltage difference across primary winding 110 A is now reversed,with capacitor 300 discharging voltage and current through primarywinding 110 A to ground through point C. As capacitor 300 discharges,the voltage difference across primary winding 110 A decreases.

A brief period during which signals from both pulse generators 230 A andB are low is inserted between alternately switching MOSFETs 210A and Bso that the MOSFETs are never switched on simultaneously. Thus, forexample, waveforms 5A-5B and 5C-5D represent first and second pairs ofpreferably orthogonal waveforms. This prevents connecting DC voltagesource directly to ground and damaging the DC voltage source. Whensignals 230A and 230B are low, the waveform across the primary winding110A will be either positive or negative depending upon the magnetizingcurrent of the primary winding. The magnetizing current of the primarywinding will flow through the body diodes of two diagonal MOSFETs. Thebody diodes are inherent to each MOSFET and connected in antiparalleltherewith. The direction and magnitude of the magnetizing currentdepends upon the magnetizing inductance, operating duty cycle and thephase of the transformer. Preferably, the time period is small and willprevent the preparation of a low output ripple voltage.

The interoperation of pulse generators 230 A and B with MOSFETs 210 Aand B creates a saw-tooth like waveform shifted downwards across primarywinding 110 A as illustrated in FIG. 5E. The general shape of thewaveforms is caused by the charging and discharging of capacitor 300 aswell as the impedances of MOSFETs 210 A and B and primary winding 110A.Input pulse generators 230 C and D and MOSFETs 210 C and D interoperatein the same manner as input pulse generators 230 A and B and MOSFETs 210A and B but phase shifted by 180 degrees to generate the waveformillustrated in FIG. SF across primary winding 110 B.

The sharing of the DC blocking capacitor 300 is an advantage of AC-DCconverter 50. For example, in the event that the DC blocking capacitoris being charged through winding 110 A and being discharged by winding110B, the RMS current flowing through the blocking capacitor ispreferably reduced. Thus, as compared to other voltage sources, asmaller or lower quality capacitor may be used.

Signals are passed from primary windings 110 to secondary windings 120where they are rectified by diodes 400 so that only the positiveportions of the signals remain. Signals from each power conversion unitsare then overlaid at node B resulting in the signal depicted in FIG. 5G.Specifically, before a first signal from one power conversion unit candecay to any significant extent, a second signal from a second powerconversion unit is overlaid on top of the first signal, resulting in aDC signal with minimum voltage and current ripples at node B as well asat the converter output. Overlaying of signals from a plurality of powerconversion units also allows each power conversion unit to contributecurrent at node B, increasing current level of the signal at the nodeand, thus, at the converter output. The signal at node B is furtherfiltered by a LC low-pass filter formed by inductor 500 and capacitor600 to further reduce voltage ripples, yielding a DC signal depicted inFIG. 5I. The rectifiers may be reversed to obtain a negative DC outputvoltage.

The amplitude of the DC output voltage is determined by the duty cycleof the pulses generated by pulse generator 230. Increasing the on dutycycle increases the average DC voltage of the pulse generators and,hence, increase the voltage across the DC blocking capacitor. With ahigher voltage across the DC blocking capacitor 300, the potentialdifference across primary windings 110A and B is smaller when primarywindings 110 A and B are connected to DC voltage source 240, decreasingthe amplitude of the signal at node B and, thus at the output.

AC DC converter 10 or 20 may be modified to eliminate sharp voltageedges and undesired voltage spikes caused by parasitic or leakageimpedance in transformers 100 or other circuit elements. For example,snubber circuits may be added across rectifying diodes 400, secondarywinding 120, or across low pass filter 500 and 600.

Referring to FIG. 6, an embodiment of the present invention usingsinusoidal input signals 200′ is shown. Referring to FIG. 7, anembodiment of the present invention using a single source 201 to providea plurality of time varying input signals 200″ is shown.

The present invention is not to be limited in scope by the specificembodiments described herein, which are intended as single illustrationsof individual aspects of the invention, and functionally equivalentmethods and components are within the scope of the invention. Indeed,various modifications of the invention, in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appended claims.

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

What is claimed is:
 1. A device for preparing a direct current (DC)signal from a plurality of input signals, comprising: a plurality oftransformers, each transformer having first and second input connectionsand first and second output connections; a respective source of timevarying input signals connected to the first input connection of eachtransformer, wherein (a) each time varying input signal alternatesbetween a positive and negative voltage and (b) time varying inputsignals connected to different first input connections have differentphases; an overlaying circuit for combining signals output by thetransformers, the output signal of each transformer comprising anasymmetric time-varying signal having a respective different phase, tothereby prepare the DC signal, the overlaying circuit comprising: afirst node to which the first output connection of each transformer isconnected, wherein an electrical path between the first outputconnection of each transformer and the first node includes a rectifier;a second node to which the second output connection of each transformeris connected; and first and second output terminals respectivelyconnected to the first and second nodes.
 2. The device of claim 1,comprising a capacitor in series with the first input connection of eachtransformer.
 3. The device of claim 1, wherein each rectifier comprisesat least one diode.
 4. The device of claim 3, wherein the overlayingcircuit combines the signals output by the transformers afterrectification.
 5. The device of claim 3, wherein at least one diode isin series with one of the output connections of each transformer.
 6. Thedevice of claim 1, wherein the time varying input signals at leastpartially overlap with one another.
 7. The device of claim 6, whereineach time varying input signal comprises at least one of a square waveand a sinusoid.
 8. The device of claim 6, further comprising at leastone power source to provide the plurality of time varying input signals.9. The device of claim 6, wherein an absolute DC bias of the signalsoutput by the transformers is less than an absolute DC bias of thetime-varying input signals.
 10. A device for preparing a direct current(DC) signal from a plurality of input signals, comprising: an inputcircuit for preparing a plurality of intermediate time-varying signalsfrom the plurality of input signals, the input circuit comprising: a DCvoltage source; first and second sets of transistors, each member of thefirst and second sets of transistors comprising a respective gate, arespective first connection, and a respective second connection, whereinthe gate of each transistor is connected to a respective different inputsignal; a plurality of power converters, each power converter configuredto prepare an asymmetric time-varying output signal from a respectiveintermediate time-varying signal, each intermediate time-varying signalalternating between a positive and a negative voltage and eachasymmetric time-varying output signal having a respective, differentphase, wherein each power converter comprises a first and a second inputconnection, wherein (a) the respective first connection of each memberof the first set of transistors is connected to the DC voltage sourceand the second connection of each member of the first set of transistorsis in electrical communication with a respective first input of a powerconverter and (b) the respective first connection of each member of thesecond set of transistors is in electrical communication with (i) arespective first input of a power converter and (ii) a respective secondconnection of a member of the first set of transistors, and wherein theinput signal connected to the gate of each member of the second set oftransistors that is connected to a member of the first set oftransistors has a duty cycle different from that of the input signalconnected to the gate of the connected member of the first set oftransistors; and an overlaying circuit for combining the asymmetrictime-varying output signals to thereby prepare the DC signal.
 11. Thedevice according to claim 10, wherein the input signals comprise firstand second pairs of orthogonal time varying electrical signals.
 12. Thedevice of claim 10, wherein the transistors are metal-oxidesemiconductor field effect transistors (MOSFETs).
 13. The device ofclaim 12, wherein the first connection of each member of the first setof transistors is a drain biased at a DC.
 14. The device of claim 13,wherein the first connection of each member of the second set oftransistors is a drain in electrical communication with a respectivemember of the first set of transistors.
 15. The device of claim 14,wherein the input signals accepted by the first and second sets oftransistors having a different duty cycle.
 16. The device of claim 15,wherein the input signals accepted by the first and second sets oftransistors have a different phase.
 17. The device in accordance withclaim 10, wherein said input circuit comprises a plurality of MOSFETsconnected in a full-bridge configuration.
 18. The device in accordancewith claim 17, wherein said input circuit further comprises a DC voltagesource connected to a drain of each member of a subset of the MOSFETs.19. The device accordance with claim 18, wherein a respective inputsignal from a power source controls a gate voltage of each of saidplurality of MOSFETs to thereby prepare the intermediate time-varyingsignals.
 20. The device according to claim 19, wherein each input signalis a time-varying input signal.
 21. The device in accordance with claim20, wherein said input signals switch MOSFETs on and off by controllinggate voltages of said MOSFETs so as to alternatively connect said powerconverters to a DC voltage source or ground.
 22. The device inaccordance with claim 21, wherein each power converter comprises atransformer with a primary and a secondary winding.
 23. The deviceaccording to claim 22, wherein each said power converter furthercomprises a rectifying diode whose anode connects to said secondarywinding and whose cathode connects to cathode of at least one otherrectifying diode of another said power converter.
 24. A method forpreparing a direct current (DC) signal from a plurality of inputsignals, comprising: providing a circuit comprising first and secondtransformers, each transformer having first and second input connectionsand first and second output connections; inputting a time-varying inputsignal to the first input connection of each transformer, thetime-varying input signals each (i) having a respective, different phaseand (ii) alternating between a positive and a negative voltage, eachtransformer outputting a respective asymmetric time-varying outputsignal, each asymmetric time-varying output signal having a respectivedifferent phase; combining the asymmetric time-varying output signals tothereby prepare the DC signal, the step of combining comprisinginputting signals output from the first output connection of eachtransformer to a first node, wherein the step of inputting comprisesrectifying the signal output from each first output connection; andoutputting the DC signal, the DC signal being output across a firstoutput connection in electrical communication with the first node and asecond output connection in electrical communication with a second nodeto which second node the second output connection of each transformer isconnected.
 25. The method of claim 24, wherein the time-varying outputsignals are rectified prior to combining.
 26. The method of claim 24,wherein the step of preparing the time-varying output signals furthercomprises reducing a DC bias of the input signals.